1. Field of the Invention
The present invention relates to a driving circuit for a brushless motor that can detect short-circuiting of the motor.
2. Description of the Related Art
An ordinary brushless motor is widely used as a driving motor in various fields, such as a fan motor mounted in an in-house unit of an air conditioner or a steering motor of an electric power steering device. Such a brushless motor includes a permanent magnet type rotor, a stator having a plurality of driving coils, a plurality of position sensors disposed around the rotor and a driving circuit. In order to rotate the rotor, the angular position of the rotor is detected by the position sensors to cyclically and timely excite the driving coils.
As disclosed in JP-A-Hei-10-191551 or U.S. Pat. No. 6,107,926 that is a counterpart of the Japanese patent application, a current sensor is disposed in the driving circuit to detect a short-circuiting problem of the brushless motor. If a short-circuiting problem takes place, the current sensor detects an amount of current that is larger than a predetermined fixed value (e.g. 60 A in FIG. 5).
Because driving current or load current of the brushless motor changes as the electric angle of the rotor of the brushless motor changes, the current sensor may not correctly detect short-circuiting problem if the rotor is positioned at an angle (e.g. 150 degree in electric angle in FIG. 5) where the driving current becomes smaller than the predetermined fixed value. In that case, various parts or components may be damaged by a large amount of short-circuiting current before it is detected.